A zinc oxide (ZnO) varistor is produced by using zinc oxide ceramics (a sintered body) which are obtained by burning zinc oxide material powder that contains zinc oxide, bismuth oxide (Bi.sub.2 O.sub.3), manganese oxide (MnO.sub.2) and cobalt oxide (CoO) as basic additives, and various oxides to be added for performance enhancement. It has been known that the threshold voltage of the zinc oxide varistor is almost proportional to the number of grain boundaries which are present between electrodes. More specifically, the threshold voltage rises by 3V to 4V per grain boundary. Accordingly, it is necessary to produce a sintered body having ZnO particles whose average particle size is about 4 to 40 .mu.m in order to fabricate the zinc oxide varistor for a high voltage. It is necessary to produce a sintered body having ZnO particles whose particle size is about 40 to 200 .mu.m for easy handling in order to produce a zinc oxide varistor for a low voltage. In order to produce the zinc oxide varistor for a high voltage, a method of adding a growth inhibitor such as antimony oxide (Sb.sub.2 O.sub.3) to inhibit the growth of ZnO particles has conventionally been used. In order to produce the zinc oxide varistor for a low voltage, a method of adding a growth promotor such as titanium oxide (TiO.sub.2) to promote the growth of the ZnO particles has been used.
However, a sintering temperature of 1150 to 1300.degree. C. is required to obtain a high-performance zinc oxide varistor for high and low voltages. The high sintering temperature causes power to be consumed, bismuth oxide to be strongly scattered, and a furnace material or container to be wasted due to the strong scattering of bismuth oxide. Consequently, it has been desired to decrease the burning temperature. If burning is performed at a high temperature, the bismuth oxide or the like evaporates actively in the air. In addition, the bismuth oxide easily reacts with many materials, and readily erodes a lot of materials such as ceramics, for example, a furnace material, a container and the like. If the burning temperature is decreased by the blending of the zinc oxide varistor according to the prior art, the threshold voltage is rapidly increased so that the irregularity of a ZnO particle size is caused. Consequently, the non-linear resistance characteristic is degraded. Further, the life for power loading, pulse current loading or the like is shortened.
According to the method according to the prior art, bismuth oxide, titanium oxide, antimony oxide, chromium oxide (Cr.sub.2 O.sub.3) and boron oxide (B.sub.2 O.sub.3) are individually added to zinc oxide material powder. Therefore, there are a portion of the titanium oxide which reacts with zinc oxide, a portion of bismuth oxide which reacts with titanium oxide, a portion of antimony oxide which reacts with zinc oxide, a portion of bismuth oxide which reacts with antimony oxide, and the like. The boron oxide tends to form a liquid phase and coalesce at the early stage of the temperature increase so that irregularity is easily caused. As a result, the sintered body has a portion in which grain growth is not promoted and a portion in which the grain growth is promoted. In the method according to the prior art, thus, it is difficult to produce a sintered body having a regular particle size.
Furthermore, it is hard to fully control the abnormal grain growth of ZnO. Therefore, there is a problem that the electric characteristics and reliability of zinc oxide varistors obtained from a given producing lot (batch) have great variation (which occurs within the lot). In addition,there is a problem that the electric characteristics and reliability of zinc oxide varistors obtained from different producing lots have great variation (which occurs between the lots).
As described above, it is impossible to stably produce a zinc oxide varistor having excellent electric characteristics and reliability with low-temperature sintering by the method using the ceramics according to the prior art.
It is an object of the present invention to provide zinc oxide ceramics for producing, in high yield, zinc oxide varistors having reliability and electric characteristics such as non-linear resistance characteristics with low-temperature sintering.